Refractive index dispersion and anisotropy in NaGd(WO 4) 2 single crystal

Abstract

Refractive index characteristics were measured in the tetragonal, scheelite-like, NaGd(WO 4) 2 (NGW) single crystal. The room temperature ordinary refractive-index dispersion was measured by the minimum-deviation method between 320 and 1342 nm. The refractive-index anisotropy was measured between 400 and 1860 nm using a combination of bireflection in an oriented single crystal prism, and birefringence modulation in an oriented plane-parallel plate. The crystal is positive uniaxial, with its optic axis parallel to the crystallographic c-axis. The dispersion was fitted to a simple two-parameter Sellemeier function. The anisotropy dispersion was also fitted to an empiric function. Taking the 632.8 nm wavelength (the He–Ne laser line) as a representative of the visible range, the ordinary refractive index is n o = 1.989(1), and the anisotropy ( n e − n o) = 0.00342(1). The refractive indices’ changes with temperature were measured between room temperature and ∼200 °C at λ = 632.8 nm, yielding d n o/d T = − 10.70 × 10 −5 °C −1 and d n e/d T = − 4.17 × 10 −5 °C −1.